This invention relates to direct current drive motor systems adapted to selectively drive or electrically retard heavy mechanical loads, such as traction vehicles, and more particularly, to an improved arrangement for energizing motor field during motoring and braking modes.
Electric drive systems for high inertia loads, such as traction vehicles, must be designed to propel the load, and to electrically retard (commonly termed "electrically brake") the load in accordance to predetermined torque vs. speed relationships and other selected parameters. Direct current motors having armature and field windings commonly are controlled by modification of their armature and field flux. U.S. Pat. No. 3,515,970, for example, discloses an arrangement for selectively controlling both armature current and field flux.
The propulsion torque of a d-c motor is proportional to the product of armature current and field strength. During the propulsion, i.e. motoring mode, maximum torque control can thus be effected by separately controlling the armature and field currents of d-c motors. At low motor speeds the counter emf is very low resulting in high armature current. Armature current at low speeds can be limited to acceptable levels by controlling the voltage applied to the armature, such as by variable impedance means connected serially between the source and the armature. Chopper switching circuits are commonly utilized for this purpose, the chopper switch being periodically commutated so that its duty cycle is variable so as to be inversely proportional to the effective impedance to be inserted serially with the armature. Such chopper circuits also commonly utilize unilaterally conducting means, termed "free wheeling" diodes, coupled in parallel with the armature circuit, e.g. armature winding and motor reactor, and poled to conduct circulating armature current during intervals when the chopper switch is cut off. Sufficient starting torque can be attained, even with separately excited motors, by application of sufficient field current. Thus control of both armature voltage and field current permits attainment of adequate torque characteristics over a wide speed range.
Control of electric retardation can similarly be obtained by armature and field control. During electric retardation, the motor acting as a generator provides armature current to a retarding, i.e. dissipative, load, which in the case of dynamic retardation, commonly termed "dynamic braking," comprises a resistance load, and in the case of regenerative retardation, commonly termed "regenerative braking" constitutes the energy source of the motor. The co-pending application Ser. No. 433,409 of E. F. Weiser now (U.S. Pat. No. 3,866,098), explains how retardation is attainable by controlling both armature current and field flux. In chopper systems electric retardation may be attained by coupling the motor armature circuit in shunt with the dissipative load, e.g. dynamic braking resistance or d-c source. The chopper switch is commonly coupled in parallel with the armature circuit so that variation of the chopper duty cycle controls armature current over a range of speeds and the resulting range of armature voltage.
Drive systems of the type discussed must be readily convertible between the motoring, i.e. propulsion, mode and the retardation, i.e. braking, mode. This requires a change of operating condition such as reversal of the field or of the armature connection. This has, for example, been accomplished by switching the armature terminals so as to reverse armature polarity in respect to field polarity. However, excessive currents occur if the switching time is not precisely controlled so as to occur, for example, prematurely during an interval when motor current flows. Alternatively, mode switching has been accomplished by switching the field terminals so as to reverse motor field. Such switching must be accomplished so that adequate field current is built up to overcome remnance motor flux. In general, switching the commonly utilized self-excited series d-c machines from motoring to braking is subject to temporary voltage transients, resulting from the change of inductive armature currents, and in delays in affecting mode switching.